Need RANKX to exclude Blanks

In the RANKX function, don't use the whole table as the first argument but FILTER it by the cells/entries that don't have BLANKs. That's also simple. And if the cell is BLANK, just don't calculate the rank. Something like:

[Ranking] = 
var AllItemsNonBlank = 
    filter(
        // Due to ALLSELECTED this is a relatve
        // ranking. If you want an absolute ranking,
        // use ALL( T[Item] ) instead.
        allselected( T[Item] ),
        not isblank( [your measure to rank with] )
    )
var Result =
    if( not isblank( [your measure to rank with] ),
        rankx(
            AllItemsNonBlank,
            [your measure to rank with]
        )
    )
return
    Result